1. Field of the Invention
The present invention relates to a reciprocating compressor, and more particularly, to an operation control method of a reciprocating compressor that is capable of stably driving a compressor when a motor is overloaded.
2. Description of the Background Art
In general, a reciprocating compressor is a device that variably controls a cooling capacity discharged therefrom by varying a compression ratio according to a stroke voltage applied thereto.
The general reciprocating compressor will now be described with reference to FIG. 1.
FIG. 1 is a block diagram of the construction of an operation control apparatus of the general reciprocating compressor.
As shown in FIG. 1, an operation control apparatus of the general reciprocating compressor includes: a reciprocating compressor (R.COMP) 12 for receiving a stroke voltage provided to an internal motor (not shown) according to a stroke reference value set by a user to control a vertical movement of an internal piston (not shown); a voltage detecting unit 30 for detecting a voltage applied to the reciprocating compressor 12 as the stroke is varied; a current detecting unit 20 for detecting a current applied to the reciprocating compressor as the stroke is varied; a microcomputer 40 for calculating a stroke by using the voltage and the current detected from the voltage detecting unit 30 and the current detecting unit 20, comparing the calculated stroke value with the stroke reference value, and outputting a corresponding switching control signal; and an electric circuit unit 10 for switching on/off an AC power with a triac (Tr1) according to the switching control signal of the microcomputer 40 so as to control a size of the stroke voltage applied to the reciprocating compressor 12.
The operation of the operation control apparatus of the conventional reciprocating compressor constructed as described above will now be explained.
In the reciprocating compressor 12, a piston is vertically moved by a stroke voltage inputted from the motor (not shown) according to a stroke reference value set by a user, and accordingly, a stroke is varied to thereby control a cooling capacity.
The stroke signifies a distance that the piston is reciprocally moved in the reciprocating compressor 12.
A turn-on period of the triac (Tr1) of the electric circuit unit 10 is lengthened by the switching control signal of the microcomputer 40, and as the turn-on period is lengthened, a stroke is increased.
At this time, the voltage detecting unit 30 and the current detecting unit 20 detect a voltage and a current applied to the reciprocating compressor 12 and apply them to the microcomputer 40, respectively,
The microcomputer 40 calculates a stroke by using the voltage and the current detected by the voltage detecting unit 30 and the current detecting unit 20, compares the calculated stroke with the stroke reference value, and outputs a corresponding switching control signal.
If the calculated stroke is smaller than the stroke reference value, the microcomputer 40 outputs a switching control signal to length the ON-period of the triac (Tr1) to thereby increase the stroke voltage applied to the reciprocating compressor 12.
If, however, the calculated stroke is greater than the stroke reference value, the microcomputer 40 outputs a switching control signal to shorten the ON-period of the triac (Tr1) to thereby reduce the stroke voltage applied to the reciprocating compressor 12.
As for the motor (not shown) installed in the reciprocating compressor 12, a coil is evenly wound thereon at a certain coil winding ratio, so that when a current according to the stroke voltage is applied to the coil, a magnetic pole is generated at the electromagnet in the coil of the motor and a magnetic flux is generated at the coil.
The reciprocating compressor is mechanically resonated at a rated driving frequency.
For example, if a rated frequency of the reciprocating compressor is 60 Hz, a resonance frequency is designed to be also 60 Hz at a rated current.
In case of a rated load of the reciprocating compressor, the resonance frequency (a rated driving frequency) is obtained by the sum of an inertia force (M{umlaut over (X)}(t)), a damping force (c{dot over (X)}(t))and a restitution (kX(t))of a spring.
f(t)=xcex1i(t)=M{dot over (x)}(t)+c{dot over (x)}(t)+kx(t)xe2x80x83xe2x80x83(1) 
k=ks+kgxe2x80x83xe2x80x83(2) 
wherein f(t) is a force applied to the motor, xcex1 is a motor constant, I(t) is current, x(t) is displacement, xe2x80x98Mxe2x80x99 is a moving mass, xe2x80x98cxe2x80x99 is a damping constant, xe2x80x98kxe2x80x99 is a spring constant, ks is a machine spring, and kg is a gas spring.
The spring constant (k) is a sum of the machine spring (ks) connected to a mass moving by the motor so as to adjust a resonance point of the reciprocating compressor and the gas spring (kg) varied depending on a load of the reciprocating compressor.
The displacement (x(t)) is a distance that the magnet is moved from the center of the coil.
By Laplace transforming equation (1), a relation between the current and the displacement of the reciprocating compressor can be obtained.
The reciprocating compressor is designed such that the resonance frequency and the driving frequency are the same with each other at a rated load.
Equation (1) can be expressed as the frequency domain as follows:                               F          ⁡                      (                          j              ⁢                              xe2x80x83                            ⁢              ω                        )                          =                                            -              M                        ⁢                          xe2x80x83                        ⁢                          ω              2                        ⁢                          X              ⁡                              (                jω                )                                              +                      c            ⁢                          xe2x80x83                        ⁢            j            ⁢                          xe2x80x83                        ⁢            ω            ⁢                          xe2x80x83                        ⁢                          X              ⁡                              (                jω                )                                              +                      kX            ⁡                          (                              j                ⁢                                  xe2x80x83                                ⁢                ω                            )                                                          (        3        )                                                      X            ⁡                          (                              j                ⁢                                  xe2x80x83                                ⁢                ω                            )                                            F            ⁡                          (                              j                ⁢                                  xe2x80x83                                ⁢                ω                            )                                      =                  1                                                    -                M                            ⁢                              xe2x80x83                            ⁢                              ω                2                                      +            k            +                          j              ⁢                              xe2x80x83                            ⁢              ω              ⁢                              xe2x80x83                            ⁢              c                                                          (        4        )                                          f          n                =                              1                          2              ⁢              π                                ⁢                                    k              M                                                          (        5        )                                ω        =                              2            ⁢            π            ⁢                          xe2x80x83                        ⁢            f                    =                                    k              M                                                          (        6        )                                          M          ⁢                      xe2x80x83                    ⁢                      ω            2                          =        k                            (        7        )                                                      X            ⁡                          (              jω              )                                            F            ⁡                          (                              j                ⁢                                  xe2x80x83                                ⁢                ω                            )                                      =                              1                          j              ⁢                              xe2x80x83                            ⁢              ω              ⁢                              xe2x80x83                            ⁢              c                                =                                    -              j                        ⁢                          1                              c                ⁢                                  xe2x80x83                                ⁢                ω                                                                        (        8        )            
wherein xcfx89 is a driving frequency (rad/s), xe2x80x98fxe2x80x99 is a driving frequency (Hz), xe2x80x98jxe2x80x99 is an imaginary number, and fn is a resonance frequency.
At this time, F(jxcfx89) is a value obtained by Fourier transforming f(t) of equation (q) and XO(jxcfx89) is a value obtained by Fourier transforming x(t).
By applying equation (5) related to the resonance frequency (rated driving frequency) of the reciprocating compressor to equation (4) related to the force and the displacement of the reciprocating compressor, a force and a displacement according to the resonance frequency of the reciprocating compressor can be obtained.
Thus, as shown in equation (8), a force and a displacement exhibits a 90xc2x0 phase difference. In addition, since the force and the phase of current are the same, a magnetic flux of the core generated by the current shows 90xc2x0 phase difference from the magnetic flux generated due to the displacement of the magnet.
This will now be described in detail with reference to FIG. 2.
FIG. 2 illustrates waveforms showing a relation between the current applied to the reciprocating compressor and a displacement in resonating at a rated load.
As shown in FIG. 2, when current is applied to the motor in resonating at a rated load, current is applied to the coil of the motor and a magnetic flux is generated at the coil in a direction that the current is applied.
As indicated by xe2x80x98axe2x80x99 shown in FIG. 2, when current is inputted counterclockwise, N pole is generated from the right side of the coil while S pole is generated from the left side of the coil. At this time, a magnetic flux generated by the current is maximized. When the magnetic flux by the current is maximized, the magnetic flux by the current and the magnetic flux according to the displacement of the magnet have the 90xc2x0 phase difference, so that the magnet is positioned at the center of the coil and the magnetic flux of the core by the magnet is minimized.
Subsequently, as indicated by xe2x80x98bxe2x80x99 shown in FIG. 2, when the magnet is moved in one direction, the magnetic flux of the core by the current is minimized, so that the magnetic flux of the core by the current almost dies down and the magnetic flux of the core according to the magnet is maximized.
When the magnet is moved back to the center of the coil, the magnetic flux of the core by the current becomes great and the magnetic flux of the core bythe magnet is minimized (as indicated by xe2x80x98cxe2x80x99 in FIG. 2).
If the magnet is moved in the opposite direction again, the magnetic flux of the core by the current becomes small and the magnetic flux of the core bythe magnet also becomes small (as indicated by xe2x80x98dxe2x80x99 in FIG. 2).
The above operations are repeatedly performed, so that the magnetic flux of the core of the motor, that is, the magnetic flux of the core bythe current and the magnetic flux of the core bythe magnet are added to have 900 phase difference.
However, during the above operation, if the compressor is overloaded, the rigidity of the gas spring is increased and a natural frequency of the reciprocating compressor becomes higher than the driving frequency, and accordingly, the current will be easily saturated.
This will now be described in detail with reference to FIG. 3.
FIG. 3 illustrates waveforms showing a relation between an input current and a displacement in case of an overload in accordance with the conventional art.
In case that the motor is overloaded, that is, if a driving current is greater than by about 1.3 times than a rated current, the rigidity of the gas spring is increased, that is, for example, the natural frequency becomes 62 Hz when the driving frequency is 60 Hz, so that a resonance point is heightened.
That is, if the driving frequency is constant and a load is increased during the operation of the motor, the value of the gas spring constant (kg) among the value of the spring constant xe2x80x98kxe2x80x99 of equation (4) is increased.
If the value xe2x80x98kxe2x80x99 is increased, Mxcfx892 of the driving frequency becomes smaller than xe2x80x98kxe2x80x99, so that the force and displacement of the reciprocating compressor have a phase close to 0xc2x0.
In other words, when the load value of the gas spring is increased, an input current is increased in order to constantly move the piston of the reciprocating compressor. Thus, as the input current is increased, the magnetic flux of the input current and the magnetic flux of the magnet have the same phase, and thus, the self-saturation becomes more severe.
In case of the overload as described above, the relation between the force and the displacement can be expressed by equation (8) as follows:                                                         X              ⁡                              (                                  J                  ⁢                                      xe2x80x83                                    ⁢                  ω                                )                                                    F              ⁡                              (                                  J                  ⁢                                      xe2x80x83                                    ⁢                  ω                                )                                              ≈                      1            k                          ⁢                  
                ⁢                  (                                    If              ⁢                              xe2x80x83                            ⁢              M              ⁢                              xe2x80x83                            ⁢                              ω                2                                       less than                           k              ,                              xe2x80x83                            ⁢              c                         less than             k                    )                                    (        9        )            
Thus, as shown in FIG. 3, the phases of the force according to the input current and the displacement are almost the same each other. That is, the magnetic flux (displacementO generated at the core of the magnet and the magnetic flux of the core generated by the input current becomes in-phase.
As described above, in case of the overload, when the phase difference between the input current and the displacement of the magnet is 0xc2x0, the magnetic flux by the current and the magnetic flux by the magnet are added to make the saturation phenomenon of the core more serious.
If the core saturation phenomenon is severe, the reciprocating compressor fails to have a sufficient cooling capacity and the current rises excessively to cause a motor trouble.
Namely, in case of the overload, the rigidity according to the gas spring is increased and the resonance point is heightened. At this time, the input current is increased, and at the same time, the magnetic flux by the current and the magnetic flux by the magnet are operated in the same phase, so that a self-saturation become more severe.
Thus, due to the self-saturation of the motor, the inductance of the motor is reduced and current is suddenly increased to cause damage to the motor.
In an effort to solve the above problem, it is designed that the weight of the moving part, that is, the piston, is made increased, so that, in case of the overload, the phases of the magnetic fluxes bythe magnet and the current are not the same with each other.
This solution, however, has a problem that a resonance at the rated load and a resonance of the reciprocating compressor become different, causing a problem of degradation of efficiency at the rated.
Therefore, an object of the present invention is to provide an operation control method of a reciprocating compressor that is capable of being driven in case of an overload by heightening a driving frequency for driving a motor as high as a certain level higher than a rated operation frequency to offset the magnetic flux of the current and the magnetic flux of the magnet, thereby preventing a saturation phenomenon of a magnetic flux by current of a reciprocating compressor or a magnetic flux by a magnet.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a reciprocating compressor using an inverter including the steps of: measuring a current load of the motor while being operated at a rated frequency; comparing the measured load and a pre-set reference load; determining an overload if the measured load is greater than the reference load, increasing an operation frequency by as much as a certain value higher than an oscillation frequency, and performing an overload operation; and increasing a voltage applied to the motor by as much as a certain level according to the increased operation frequency and performing an overload operation, in order to compensate a stroke reduction generated as the operation frequency is increased to as high as the certain value.
The advantages in accordance with the present invention are embodied in an alternative embodiment, wherein there is provided an operation control method of a reciprocating compressor driven by an inverter comprising the steps of measuring a resonance frequency applied to a motor while the reciprocating motor is being operated at a rated frequency, comparing the measured resonance frequency with a pre-set reference resonance frequency, keeping operating the reciprocating compressor at the rated frequency if the measured resonance frequency is smaller than or the same as the reference resonance frequency, and determining an overload if the measured resonance frequency is greater than the reference resonance frequency and increasing the current operation frequency by as much as a certain level, for an overload operation.
The reference resonance frequency is set the same with the rated frequency in case of the rated load.
The overload is a value set by an experiment, for which a driving current value is greater by over 1.3 times (30%) than the current value at the rated load.
In case of the overload, the operation frequency is increased by a certain value higher than the resonance frequency, for the overload operation.
As for the operation frequency in case of the overload, a current is set greater by 1.3 times (30%) than the rated current, so that a phase difference between a magnetic flux generated by the input current and a magnet flux generated by the magnet is 180 degree.
In case of the overload, if the operation frequency is increased by a certain value, it is moved in the same direction as the pole generated in the coil of the motor.
If the operation frequency is increased by as much as a certain value, the current inputted to the motor and the magnetic flux of the magnet are moved in a direction that they are mutually offset.
In case of the overload operation, a voltage of the motor is increased by a certain level in order to compensate a stroke reduction according to the increase in the operation frequency.
The overload operating step comprises comparing the waveform of the input current applied to the motor with a reference current sine waveform, and determining an overload if a distortion occurs to the waveform, and increasing the current operation frequency by as much as a certain level, for an overload operation.
Alternatively, the overload operating step comprises comparing a power applied to the motor with a reference power, and determining an overload if the applied power is higher than the reference power, and increasing the current operation frequency by as much as a certain level, for an overload operation.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.